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Sunday, March 27, 2011

Avonside (March) - factual geotech report

[NOTE: this post is now nearly 3 months old]  EQC released the full factual report on Avonside (you can get it here). EQC's definition of Avonside is shown in the map:

  This report is full of technical information aimed at geotech specialists. While most of it has little meaning for us, I did find the site (land) description, and the last section on the Regional Council boreholes, quite fascinating.

Descriptions of the land

The following is an extract from the Tonkin and Taylor report (p 2)
3.2 Regional geology

The Geology of the Christchurch Urban Area (Ref 3) shows that the subject area of Avonside is underlain by alluvial sand and silt overbank deposits around the banks of the Avon River. The centre of the subject area is shown to be underlain by sand of fixed and semi-fixed dunes and beach deposits of Holocene age.
The reference to the dunes and beach deposits being of Holocene age doesn't give much information: we are living in the Holocene age, which started around 12,000 years ago, and is still going. The bottom line seems to be that in parts of our area we have built and live atop sand.

This is probably a surprise to many, however it was well known in the 19th century through the "black maps" which were compiled by surveyors in the 1850s. Copies of some of these maps are held in the New Zealand Room of the Christchurch Public Libraries. Since they were first drawn these maps have been used by the council and others (e.g. the now defunct Drainage Board). The map below is an enhanced extract from a larger map on the Council's website (get a copy here).

As you can see, most of Avonside was low-lying with fern and tutu, swampy areas, sand hills and a bit of terrace high ground. Woodham Road, named in green, runs along the bottom of the map. Click on the map to enlarge it.



Looking Underneath

The Regional Council, some time past, sank bores in our area. Two of them allow a glimpse of what lies just below the surface, and how different it is between the two points. I have no idea if the changes described below are representative of the area in general, but for now it helps me build up a picture of the land.

Key pages, if you want to check it out, are: Appendix A, second map (the map of where the ground samples were made), Appendix G: Environment Canterbury Boreholes (the Borelogs for well M35/3814 and M35/14578).

Well M35/3814 is behind the main wing of Avonside Girls High School and well M35/14578 on Retreat Road near Keller Street (see the map below). Looking at the first few metres of each well there is a significant difference in what was found. At AGHS the first 2.40 metres consists of clay followed by 4.6 metres of brown shingle. On Retreat Road there is 91cm of clay, followed by 4.3 metres of blue sand then a bit of gravel (the well only goes to 5.32 metres). These two points are about 350 metres apart.



Would any of us, knowing this information, have taken up an opportunity to build on this land? How many would want to rebuild on it and, if we do, what would it take to make the land sound?

NOTE added 28 April: changed the introduction to ensure no one thought this post referred to the much sought after May 2011 information release.
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